A lot of things come at no cost though. I find it amazing how many people for example will spend a fortune on their graphics card, motherboard, processor, ram, hard drives, etc... but then run it with a cheapo power supply.
Let's say that you're one of those (probably the majority) that leaves their computer on 24/7. Let's say your gaming computer's average power consumption, between idling and heavy usage, is maybe 200W. Let's say the power supply lasts an average 3 years. Let's say that the difference between a cheapo 75% efficient power supply and an excellent 95% efficient supply is $50. Then the better supply saves 40W on average, or 1051 kWh over its lifespan. At an average US electricity price of, what, 12 cents per kWh, that's a savings of $126. You not only help the environment, but you easily save yourself money.
It's not just power supplies that matter - the same logic can be applied to processors, graphics cards, and other hardware as well. Always check the power consumption - not just for the environment, but for your pocketbook as well. Often it saves money to spend more upfront.